1. Field of the Invention
One embodiment of the present invention relates to a light-emitting device using electroluminescence. Further, one embodiment of the present invention relates to an electronic device using the light-emitting device.
2. Description of the Related Art
In recent years, research and development have been actively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a substance with a light-emitting property is provided between a pair of electrodes. A voltage is applied to this element, so that light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is a self-luminous type, and thus a light-emitting device using the light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. In addition, such a light-emitting device also has advantages in that the device can be formed to be thin and lightweight and that response time is high.
For example, in the case where the light-emitting element is applied to an active matrix light-emitting device, the light-emitting element is formed over a substrate provided with a transistor for controlling light emission or the like in some cases. With such a structure (bottom-emission structure) where light emitted from a light-emitting element is extracted to outside through a substrate provided with a transistor for controlling light emission or the like, there is a problem of low aperture ratio due to a wiring, a transistor, or the like.
In order to solve that problem, a structure (top emission structure) where light is extracted from the side opposite to an element substrate has been proposed (e.g., see Patent Document 1). With the top-emission structure, the aperture ratio can be increased, and the extraction efficiency of light can be increased, which are advantageous in power consumption and high definition.
Further, in order to improve the extraction efficiency of light from a light-emitting element, a method in which a microcavity using a resonant effect of light between a pair of electrodes is used to increase the light intensity in a specific wavelength (e.g., see Patent Document 2), a structure in which a region where the cavity length is different is provided in each pixel to improve the viewing angle dependence of a light-emitting element (e.g., see Patent Document 3), and the like have been proposed.